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Machine Design

A CVT in a compact, efficient package

A continuously variable transmission, in theory, has an unlimited number of gear ratios between the highest and lowest settings.

But most CVTs are complex, expensive, have poor efficiencies, and aren't scalable. A new type of CVT, the NuVinci from Fallbrook Technologies Inc., San Diego (, combines toroidaltraction CVT with the versatility of the planetary gear arrangement to create a low-cost, highly efficient drive for human-powered and motorized vehicles.

While the NuVinci uses rolling traction to distribute torque like toroidal CVTs, it distributes the transmitted torque over several spheres in an inherently stable configuration. The rotating balls between the input and output section of the CVT tilt to vary transmission speed. As the balls tilt, they change their contact diameters to vary the speed ratio. This lowers contact pressures and improves durability, stability, and torque density. And since it uses a planetary arrangement, torque can be summed or divided.

Compared to conventional CVTs, the NuVinci is less complex and more efficient, which lowers manufacturing and operating costs. Acceleration is faster and smoother because the engine can run at peak performance. The transmission accepts multiple inputs while varying speed and ratio. The technology is also scalable. For example, more torque can be added to a low-torque application by adding balls. The transmission also has input and output shafts in line, making the transmission smaller and easier to package.

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